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Universal Serial Bus (USB)

The first version of USB (Universal Serial Bus) was developed in 1995 by
a consortium of NEC, IBM, Intel, Microsoft and others. The purpose was
to provide a simple link between computers and peripheral devices.

One of the main functions of the USB interface is the possibility of
connecting devices to the PC without having to restart the PC (hot
plugging). The standard provides power to devices via the bus and a
total of 127 devices can be connected via hubs. The complete data
transfer via USB is controlled by the PC and is now found on most
motherboard chipsets removing the need for a separate interface card.
It is possible, however, to expand the USB capacity of a PC by additional
host controller boards.

The bandwidth of the original USB 1.0 standard is 12 Mb/s, which was
also adopted for the revised standard USB 1.1. This throughput is only
approximately 1 MB/s, which is enough for the original target application such as that of connecting the keyboard and the mouse, but it was
not enough for applications requiring the huge bandwidth needed for
the connection of cameras. When the successor, USB 2.0, was launched
in 2001, the bandwidth increased to 480 Mb/s, which made USB 2.0
interesting for devices with higher data rates such as bulk storage, CD
burners and also for imaging and machine vision.

With an effective bandwidth between 40 MB/s (as a function of the
USB host controller) and 45 MB/s, depending on the protocol used, the
performance of USB 2.0 is approximately between IEEE 1394a and b.
Even if these two interfaces may seem very much alike at first glance,
there are a few major differences. USB uses a star topology with several
levels, i.e. several devices can be connected to node points, e.g. USB hubs.

Up to six levels with node points are permitted below the host controller. It should be noted, however, that daisy chaining of devices or PCs
can also be done in IEEE 1394, but not with USB. USB in general uses
different protocols such as bulk or isochroneous. On most USB devices
and on all current USB3 Vision cameras the bulk protocol is used which
provides guaranteed delivery but has no assigned bandwidth.

The successor USB 3.0, also known as superspeed USB, was released in
November 2008 and we have seen many devices reach the market with
2012 seeing the launch of the first machine vision cameras. USB 3.0
operates at a bandwidth of 5 Gb/s enabling net data rates of
substantially more than 350 MB/s. This places USB 3.0 roughly on a
level with CameraLink Base, but unlike CameraLink or CoaXPress, it does
not require an extra frame grabber because a USB3 interface is already
integrated on many motherboards and laptop computers. Like USB 2.0,
USB 3.0 is downwards compatible. This means that USB 3.0 devices
with a correspondingly lower bandwidth can operate on USB 2.0 ports
and USB 2.0 devices can operate on USB 3.0 ports. However, as USB 3.0
also comes with new connectors, care needs to be taken when
integrating legacy devices and USB 3.0 devices. It should be noted that
USB 3.0 is no longer compatible with the "old" USB 1.1 controllers.
Like other generic interfaces such as Gigabit Ethernet, the standard
does not define a specification for working with cameras and as such it
is necessary to install and use the software of the camera manufacturer
when using a USB camera. This creates a problem for the user when
new cameras need to be integrated, and it can provide difficulties in
embedded solutions because a camera manufacturer may not be able
to provide drivers for any conceivable operating system. If a uniform
standard existed, the integration could be handled by specialist
developers other than the manufacturer. In addition a new software
platform would have to be installed only once and could be used for
different camera manufacturers.

Some USB devices load the camera firmware from the host during the
booting phase of the camera. While this means drivers from the camera
manufacturers must be used, it does mean new features can easily be
added to a camera by simply releasing a new driver.
USB cameras in the market use different data transfer implementations.
For example data can be transferred using bulk transfer which has no
reserved bandwidth. The bandwidth must be managed on the bus by
the host controller. The difference for example to GigE is that we have
guaranteed delivery so that even if the bus is under heavy load, the
application might not receive all images, but it will only receive complete images. The advantage of bulk traffic is that many cameras can
operate on the same bus.

Alternatively, data can be transferred via isochronous channels. Isochronous channels have a fixed bandwidth
reserved for them in the software which is engaged even if no data is
transferred at a given time. In USB 2.0 the number of isochronous
channels was limited, thus limiting the number of cameras that could
be connected.